As a desulfurization facility which removes sulfur oxide in a flue gas generated by combustion of a fuel oil or coal in a thermal power station or the like, a wet type flue gas desulfurization apparatus is widely used. With the wet type flue gas desulfurization apparatus, a flue gas and a slurry absorbent (which includes a calcium compound such as limestone) are brought into contact with each other in an absorber so that sulfur oxide in the flue gas is absorbed in the slurry absorbent, and the slurry absorbent after the contact is oxidized and subjected to solid-liquid separation, with the result that gypsum is produced as a by-product.
In this case, sulfur dioxide (SO2), which is a main component in sulfur oxide in the flue gas, is absorbed in the absorbent through a reaction, and reacts with oxygen in the flue gas or oxygen supplied from the outside to produce gypsum.
The concentration of oxygen in the flue gas is low and oxidation from calcium sulfite to gypsum does not proceed sufficiently with the amount of oxygen in the flue gas. Therefore, in a wet type flue gas desulfurization apparatus, an oxygen-containing gas from the outside the system is introduced into the absorbent to promote the production of gypsum. When the amount of the introduced oxygen-containing gas is small, the concentration of unoxidized calcium sulfite increases, which results in problems such as inhibition of dissolution of calcium carbonate which is an absorbing agent, and a decrease in a desulfurization performance.
On the other hand, in order to maintain a high conversion rate from calcium sulfite to gypsum, the oxygen-containing gas has to be supplied in an excessive amount in consideration of a boiler load fluctuation and the like, which leads to an increase in running cost, and an increase in chemical oxygen demand (COD) in wastewater since the generation of peroxides such as S2O6 and S2O8 may be caused thereby. Therefore, it may be necessary to adjust the amount of the introduced oxygen-containing gas to be within an appropriate range.
Regarding a controlling method for adjusting an amount of an introduced oxygen-containing gas which contributes to oxidation of calcium sulfite, a method using an oxidation-reduction potential (hereinafter referred to as “ORP”) is known. In other words, a conventional method which controls, with an ORP, an amount of an introduced gas is a method in which an ORP set value is determined in advance based on a result of an obtained correlation between the ORP and the concentration of sulfurous acid, and an amount of an introduced gas is controlled by a deviation signal between each of signals of successively detected ORPs of an absorbent and the ORP set value.
However, for example, depending on the boiler combustion state, there may be a case where the concentration of oxygen (O2) in the flue gas becomes higher departing from the correlation, or a case where the concentration of sulfur oxide (SO2) in the flue gas becomes lower departing from the correlation. In such cases, there is the following problem. Even if the amount of oxidation air introduced into the absorbent storing unit of the wet type flue gas desulfurization apparatus is reduced to zero, sulfurous acid generated by absorption of sulfur oxide is sufficiently oxidized by natural oxidation caused by contact between the flue gas and the absorbent in the absorber. In addition, the absorbent is put into a peroxidized state by natural oxidation caused by contact between the flue gas and the absorbent in the absorber, thereby resulting in making it difficult to control the ORP to be a desired value.
For example, even in a case where an apparatus is designed such that the ORP is controlled to be an appropriate value, when the ORP fluctuates unstably between extremely high values such as 200 to 1000 mV to cause a peroxidized state, a heavy metal ion contained in the flue gas, for example, manganese, is oxidized to form manganese oxide. Due to this, there occurs a problem such as coloration of gypsum. There also occur problems such as malfunction of a pH meter, blockage of nozzles, and clogging of a solid-liquid separator caused by manganese scale deposits. In addition, there occurs problem that the absorbent could not maintain wastewater treatment standards, which necessitates a separate post-treatment, since selenium exiting in the form of tetravalent selenium (Se4+) in the absorbent is changed to the form of hexavalent selenium (Se6+) which is difficult to remove, and persulfuric acid or the like is generated in the absorbent.
Therefore, conventionally, the following has been proposed. The oxidation-reduction potential of an absorbent is calculated with an ORP meter, and a supply amount of an oxygen-containing gas is adjusted according to the oxidation-reduction potential. When the oxidation-reduction potential has increased above the range adjustable depending on the supply amount of the oxygen-containing gas, the oxidation-reduction potential is adjusted by supplying, to the absorbent, an oxidation inhibitor (silicon-based defoamer, oil/fat-based defoamer, fatty acid-based defoamer, mineral oil-based defoamer, alcohol-based defoamer, amide-based defoamer, phosphoric ester-based defoamer, metal soap-based defoamer, alcohol, and glycerin) (Patent Literature 1).